The present invention relates to a toner image fixing member, fixing roller and fixing device for use in an electrophotographic apparatus using an electrophotographic process, such as a copier, facsimile and printer, and more particularly to an electrophotographic apparatus capable of printing a full color image.
A conventional fixing device used in an electrophotographic apparatus (e.g., a copier or a printer) is usually of the heating roller type wherein an unfixed recording paper with an unfixed toner image passes through a nip between paired fixing rollers being heated to fuse and fix the toner image thereon. In particular, a full color printing electrophotographic machine necessarily uses a fixing device which has fixing rollers covered with silicone rubber, on the surface of which is applied an anti-offset agent such as silicone oil having a small surface energy on the surfaces of the rotating rollers to prevent toner from adhering to the roller surfaces (i.e., preventing so called offsetting toner to another print).
However, the silicone rubber cover of a fixing roller is not so durable and may suffer deterioration of its surface quality by friction with recording paper and toner particles, causing offsetting toner to the roller only at and after 20000 printing cycles.
Japanese Laid-open Patent Publication No. 7-219375 proposes a method for covering the roller surface with a layer made of fluororesin having an excellent durability. The roller covered with a fluororesin layer, however, can not be evenly wetted with a silicone oil because the fluororesin layer has a small surface energy and, therefore, a small wettability with silicone oil.
To solve the above-mentioned problem, two methods were recently proposed, one of which is to use silicone oil denatured to have an improved wetting ability to fluororesin and the other is to improve the wettability of a fluororesin layer of the roller with silicone oil by modifying the fluororesin layer surface itself to have a reduced surface tension.
The former method, however, was accompanied by some new problems that silicone oil denatured in respect to fluororesin is rather expensive than standard silicone oil and may produce toxic fluorine gases when it is heated at a high temperature. The latter method was accompanied by an increased surface roughness of the fluororesin surface to be easily contaminated with toner.
Furthermore, a fluororesin-covered fixing roller in comparison with a silicone-rubber-covered fixing roller has a higher hardness (less elasticity) and, therefore, is inferior in ability of following up a toner image and an uneven surface of recording paper. Consequently, the roller may not sufficiently heat a toner image in particular at edge portions thereof, resulting in poor sharpness of corresponding portions of the image.